1. Field of the Invention
The present invention relates to an image forming apparatus for ejecting a liquid from an opening formed in a liquid ejection head and forming an image on a printing medium, and a method for humidifying in ahead cap, whereby, when the liquid ejection head is not used, the circumferential edge of the opening is covered in order to protect the opening from external air.
“Printing” described in this specification includes not only the forming of meaningful information, such as characters and figures, on printing media, but also the forming of images, designs and patterns, regardless of whether the data are meaningful or whether the data are visible to a person, and the performance of a process, such as an etching process, for a printing medium.
The term “printing medium” includes not only general purpose paper used for a printing apparatus, but also liquid acceptable media, such as a fabric, resin film, a metal sheet, glass, ceramics, wood and leather, and a three-dimensional object other than a sheet member, for example, a spherical member or a cylindrical member.
The term “liquid” should be interpreted broadly as well as in the definition for “printing”, and is a liquid that can be used, by being ejected onto a printing medium, to form images, designs and patterns, to perform a process, such as an etching process, for a printing medium, or to perform an ink process, for example, a process for solidifying, or rendering insolvable, a color material contained in ink to be ejected onto a printing medium. In other words, the term “liquid” represents any kind of ink used for printing.
2. Description of the Related Art
A printing unit, incorporated in various types of printers, copiers and facsimile machines, employs image data to be printed, and prints a dot pattern image on the surface of a printing medium, such as a paper sheet or a plastic film. Such an image forming apparatus can be classified as an ink jet printing type, a wire dot-type impact printing type, a thermal printing type and a laser beam printing type, depending on the form of the printing.
Among those types, an ink jet image forming apparatus ejects liquid droplets, such as ink droplets, onto a printing medium through an opening in a liquid ejection head, and attaches the ink to the surface of the printing medium to obtain an image. Since the ink jet image forming apparatus requires only a simple configuration, this apparatus can be provided comparatively in a small size and at a low price. Therefore, the ink jet image forming apparatus is regarded as the one that can cope with the recent demand for high-quality image output for digital cameras, and the demand for the mass printing of documents, and its use has spread rapidly.
There are some ink jet image forming apparatuses that use a liquid ejection head wherein a plurality of openings are integrated and arranged in order to increase the printing speed, or that use a plurality of liquid ejection heads to perform color printing. Furthermore, to cope with requests for high-resolution images or high-quality images, downsizing the openings and increasing the density of the openings have tended to be accelerated for the liquid ejection head. Further, to increase small printer usability, with the appearance of Li ion batteries, portable printers have begun to be sold on the market. In, for example, Japanese Patent Application Laid-open No. 9-213359 (1997), a portable printer is disclosed that to perform printing also employs a fuel cell so there is no need to be concerned about the power remaining in a cell.
For portable ink jet printers or high-quality and high-performance printers, the downsizing of openings has increased, and the range of the temperature and humidity for the use of printers, including outdoor use, has been extended. Therefore, a countermeasure for drying openings has become more important. When an opening is dried, for example, water present in a liquid path communicating with the opening evaporates, the viscosity of a liquid such as ink is increased, and the solidified liquid is attached to the inner wall of the liquid path. As a result, liquid ejection can not normally be performed, and in the worst case, the liquid ejection head must be exchanged.
Thus, in order to smoothly eject liquid droplets from an opening, there is a proposal according to which suction is applied to periodically remove liquid droplets from am opening to outside a liquid ejection head, and to discharge very viscous liquid and solidified liquid attached to the inner wall of a liquid path. Another proposal is disclosed in Japanese Patent Application Laid-open No. 10-251484 (1998), according to which water is sprayed near an opening in a liquid ejection head.
In a situation wherein an ink jet printer is connected to a home personal computer and a user employs software to shut down the OS of the personal computer and turns off the ON/OFF switch of a power outlet to reduce power consumption, the supply of power to the printer connected to the power outlet is cut off. Therefore, for a printer wherein a cap member closely covers the circumferential edge of the opening when the printer is not is use, supply of power may be cut off before the cap member has covered the opening. In this case, the opening is not fully protected by the cap member and may be dried, and it may be difficult for liquid to be normally ejected from the opening the next time the printer is used.